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BMW K1200S Drivetrain
source: BMW Press material
Second generation of BMW four-cylinder motorcycle engines--radically new and with unique technical solutions.
Fitted crosswise, the straight-four power unit featured in the BMW K 1200 S displaces exactly 1,157 cc. Maximum output is 123 kW (167 bhp) at 10,250 rpm, maximum torque of 130 Nm (96 lb-ft) comes at 8,250 rpm - with more than 70 per cent of the engine's maximum torque available from just 3,000 rpm.
Sporting riding characteristics with rideable power easy to control at all times was BMW Motorrad's clear-cut objective in developing the new machine. Weighing just 81.3 kg (179.3 lb) (including the clutch and gearbox), the engine is one of the lightest engines of its size in the market.
A well-conceived overall configuration, plus space-saving arrangement of auxiliary systems and the integrated gearbox add up to provide a very compact drivetrain with ideal concentration of masses in the middle of the motorcycle. On the level of the crankshaft, the K 1200 S is just 430 millimetres or 16.93 wide, placing the engine of the K 1200 S closer to current 600-cc power units than to the usual engines in the +1,000-cc segment.
A wide range of exceptional and innovative details and solutions with design concepts carried over from the latest motorsport technology add up to create a unique drive system once again proving BMW's unchallenged leadership in the production of high-performance engines. And like in Formula 1, it is not individual features and qualities which count that much, but rather the perfect interaction of technical solutions and the optimum integration of the chassis and suspension.
BMW's engineers have also overcome the inherent disadvantage of the relatively high centre of gravity of the engine with a conventional four-cylinder by applying a solution with that typical BMW ingenuity: They have intelligently built on a well-known principle, consistently enhancing its design and concept to an even higher standard by tilting the cylinder axis on the engine of the K 1200 S to the front by 55. This not only ensures a low centre of gravity, but also provides the desired load on the front wheel - just right for a sporting style of riding with precise feeling for the road and clear feedback from the front. This greater angle also provides space for an aerodynamically efficient intake system directly above the engine and ensures an ideal flow of power in the frame profiles.
This second generation of four-cylinders in the more than 80-year history of BMW Motorrad again lives up to BMW's principle of offering demanding and unique solutions far superior to the usual standard. So it is fair to say that in its concept and design, the engine of the K 1200 S must currently be the most advanced and consistently engineered power unit of its kind in the world of motorcycles.
Crank drive and engine block - slim and slender despite 1,200 cc.
The crankshaft of the K 1200 S power unit is forged out of one piece of heat-treated steel. It comes with 8 counterweights and the usual crank angle of 180 for consistent firing intervals.
The stroke:bore ratio ensures adequate overlap of the bearing journals in the interest of superior stiffness. The crankshaft runs in anti-friction bearings, both the main and conrod bearing journals measuring 38 millimetres or 1.50 in diameter. The principle of supplying oil to the crankshaft and bearings has been carried over from Formula 1. Instead of feeding lubricant the conventional way in a radial flow via the crankcase to each main bearing and from there via a ring groove to the bores on the conrod bearings, lubricant for the bearings is fed directly into the crankshaft in an axial flow. From there the lubricant flows through holes within the crankshaft, on to the conrod bearings. This makes the usual ring groove superfluous and keeps the bearing slimmer and more compact, even though it carries the same load on the same width otherwise occupied by the groove. The advantage is adequate freedom of design for a very short and compact crankshaft and minimum distances between cylinders - which in turn keeps the overall engine slender and compact.
A further advantage of this oil supply concept is that the oil entering the crankshaft no longer acts against centrifugal forces, but is on the contrary supported by such forces. This allows the oil pump to run at a much lower pressure, pump characteristics and the supply volume being optimised for minimum power loss. Oil supply to the main cylinders, finally, follows the usual pattern via the main oil pipes in the crankcase, from where oil supply bores lead to the bearings.
Two of the crankshaft counterweights serve as gears for the primary clutch drive and, respectively, for driving the two balance shafts. The other counterweights are perfectly designed for smooth flow dynamics, the ratio between mass and inertia radius being optimised in a precise calculation process.
The camshafts in the cylinder head are driven by a chain running on a sprocket bolted on to the right end of the crankshaft. Running in anti-friction bearings, the connecting rods are light forged components made of heat-treated steel. Measuring 120 millimetres or 4.72 in length, they interact with the short stroke of the engine to enhance the already high standard of running smoothness.
Together with the anti-friction bearings, the connecting rods weigh a mere 413 grams. Again taking up a typical BMW concept, the upper conrod opening comes complete with a bearing bush for a running life of more than 100,000 kilometres or 60,000 miles. The connecting rods themselves are split horizontally by proven crack technology breaking through the large conrod opening specifically on the middle level by means of a tensile force applied suddenly and instantaneously in a hydraulic process. The fracture surface provided in this way subsequently allows extremely accurate assembly without any further centring of the components.
The engine comes with proven lightweight box-type pistons featuring an extra-short piston skirt and three rings (two compression and one oil removal ring). Despite the high compression ratio, the piston base and valve pockets are flat in configuration thanks to the flat curvature of the combustion chamber. This enhances the thermodynamically efficient combustion process and allows weight-optimised contours on the bottom of the piston.
Piston weight complete with the pins and rings is just 299 grams. To dissipate heat on the piston base surface subject to high thermal loads, the surfaces are cooled by oil injection nozzles in the crankcase serving to extend service life. To eliminate second-order free mass forces inevitable on a straight-four power unit, the crankcase, operating via a tooth belt, drives two balance shafts positioned below the crankshaft. This ensures 100 per cent balance and force compensation, the balance shafts running in roller bearings being spaced out symmetrically in front of and behind the crankshaft to avoid any additional mass momentum.
The balance shafts run at twice the speed of the crankshaft. To minimise the dissipation of sound waves and noise, the balance weights are connected via elastomer elements to the balance shafts.
Horizontally divided cylinder crankcase in open-deck design.
The two-piece crankcase is made of high-strength aluminium alloy, with the two sections split down the middle of the crankshaft. Die-cast, the compact upper section forms an extremely stable structure made up of the four cylinders and the upper bearing frame for the crankshaft. The cylinder block complete with the water jacket is in open-deck design, the cylinder liners are finished with a wear-proof, low-friction nickel-silicon dispersion coating. The lower section made as a pressure-casting forms the counterpart for the main crankshaft bearing and serves as the mounting point for the gearbox. Cylinder head and valve drive based on Formula 1 design and construction principles.
The output, performance features, quality of combustion and, accordingly, fuel economy of an engine depend to a large extent on the cylinder head and valve drive. In its design and construction, therefore, the four-valve cylinder head of the K 1200 S is conceived to ensure optimum duct geometry, compact dimensions, optimum thermodynamics, and a reliable thermal balance. The single design feature absolutely crucial to optimum design and configuration of the cylinder head is the valve angle. A particularly tight or narrow valve angle, for example, will provide an ideally straight intake duct and compact combustion chambers for high compression and optimum efficiency.
The criteria crucial to valve drive and, accordingly, optimum output at smooth and unproblematic engine speeds not affecting the valve drive are stiffness, the minimisation of moving masses, and maximum overlap on the valves themselves. In choosing the configuration of the K 1200 S power unit, BMW's engine experts have opted in favour of follower arm timing with two overhead camshafts. This offers the perfect combination of maximum stiffness and minimum weight on the moving valve drive components, together with compact design of the cylinder head. Indeed, it is no coincidence that this principle has in the meantime become the usual standard on today's Formula 1 power units, BMW Motorrad's engine specialists therefore being able to capitalise in this case on all of BMW's experience in motorsport in terms of lubrication and surface coating.
The valve angle on the K 1200 S power unit is 10 on the intake and 11 on the outlet side - figures so far not offered by any of the competition's engines in the market. Out of the two overhead camshafts, in turn, only the outlet camshaft is driven by a chain from the crankshaft, the intake camshaft being driven by gear drive from the outlet shaft. The big advantage is that this requires only one sprocket on the cylinder head, the entire concept thus providing even greater precision in valve timing and keeping the width of the cylinder head to a minimum.
The camshafts are located directly above the valves. The overall layout of the cylinder head allows an ideal 1:1 transmission ratio on the follower arms, thus keeping flexural tension to an absolute minimum and allowing the use of extremely delicate, lightweight levers. The speed limit chosen for practical purposes on production models is 11,000 rpm, while in mechanical terms the parts and components are able to take and withstand far higher speeds.
Valve diameter is 32 mm or 1.26" on the intake and 27.5 mm or 1.08" on the outlet side.
Record compression ratio.
As already mentioned, the narrow valve angle keeps the combustion chambers compact, with flat upper curvature. Indeed, this is the prerequisite for a high level of geometric compression with a thermodynamically designed, largely flat piston floor. The compression ratio of 13:1 is unprecedented on a standard-production gasoline engine, clearly confirming the ideal geometry of the combustion chambers with an ideal combustion process and optimum efficiency.
Same oil supply as on a racing engine.
The K 1200 S comes with dry sump lubrication otherwise found mainly on racing engines. The advantages of this principle are supreme dependability with a steady oil flow even under extreme conditions and the flat design of the crankcase keeping the entire engine in a flat and low position and allowing a low centre of gravity. Not requiring an oil sump, the power unit is 60 millimetres or 2.4 lower down than on a conventional construction.
The oil reservoir is in the frame triangle right behind the engine. The dual oil pump driven by a chain from the clutch shaft is housed in the rear crankcase area, drawing in lubricant from the oil reservoir and feeding it under pressure initially to the oil filter (main flow filter) fitted on the lower left side of the crankcase, within easy access from outside. From there the pressurised oil flows into the main oil line in the crankcase and is spread out through internal holes leading to the lubrication points. The oil flowing back gathers at the lowest point in the crankcase in a recess within the bottom cap. The second pump delivers the oil flowing back first to the oil cooler and from there back to the oil tank through a patented reflow system.
The oil cooler is in the front fairing in a favourable flow position beneath the headlight. Consistently reflecting the overall philosophy of the machine, the oil cooler pipes are made of aluminium.
In an ingeniously simple process, the oil level is checked through a transparent, patented plastic tube outside the oil tank used in service to drain oil from the tank with an overall volume of 4.2 litres or 0.92 Imp gals.
Engine cooling - keeping a cool head and a good thermal balance.
An intelligent cooling concept ensures a perfect thermal balance within the engine. The flow of coolant between the cylinder head and the cylinders is subdivided 73:27 by appropriately dimensioned flow cross-sections. The coolant itself flows crosswise through the cylinder head, re-cooled coolant entering the cylinder head on the "hot" outlet side, meaning that thorough cooling of the cylinder head ensures rapid dissipation of heat and an optimum temperature balance precisely where the thermal load is greatest. The reduced flow of water on the cylinders enables the engine to warm up more quickly and reduces cold running wear and friction, also in the interest of enhanced fuel economy.
The water pump is mounted on the left-hand side of the cylinder head, where it is driven by the intake camshaft. This configuration and direct injection of cooling water into the cylinder head makes the usual hoses superfluous and keeps the remaining hoses leading to the radiator extremely short. With the engine requiring only two litres of coolant, weight is once again reduced even further.
The patented radiator also boasts motorsport technology: Trapezoidal and appropriately curved in design, the radiator is fitted at the front lower end of the fairing, again in the interest of a lower centre of gravity. Thanks to the high degree of efficiency and aerodynamic optimisation of both the fairing and flow conditions, a relatively small surface of only 920 square centimetres ensures adequate dissipation of heat under all conditions. The integrated thermostat, in turn, allows the engine to warm up very quickly and the dirt trap fitted in front of the radiator has been optimised for perfect aerodynamics.
Alternator and starter.
To keep the K 1200 S slender and dynamic, the electrical ancillary units as well as their drive systems are fitted behind the crankshaft in the open space above the gearbox. The alternator is driven by the primary gear on the clutch, with maximum output of 580 Watt and maximum power of 42 Amps. The layshaft starter is connected by a freewheel running on the alternator drive gear.
Power transmission - multiple-plate oil bath clutch and cassette-type gearbox.
The K 1200 S also ranks unique among production motorcycles in its gearbox technology: For the first time in the history of BMW motorcycles with shaft drive, the K 1200 S features a multiple-plate oil bath clutch (diameter of the friction plates 151 mm/5.94) and a gearbox fully integrated together with the angle drive within the engine housing. The big advantages of this configuration, of course, are compact dimensions and efficient concentration of mass weight.
Introducing this concept, BMW Motorrad has however not taken the usual approach. Instead, the gearbox is a separate, built-in unit commonly referred to as a cassette gearbox. This concept comes from motorsport, where it allows rapid and efficient replacement of parts. On a production machine it offers advantages in the assembly process, since the entire gearbox can be preassembled as one single unit.
With the gears shifted by claws, this two-shaft gearbox is slender and light. The natural process of shifting from one ratio to another is conducted by a gearshift cylinder, gearshift forks, and slide wheels providing positive engagement. To save weight the hollow gearshift cylinder is made of a high-strength aluminium alloy running in roller bearings. The shift forks are also made of aluminium and are lubricated by oil supply under pressure. A particularly smooth gearshift is ensured by anti-friction bearing bushes with a very smooth bronze/teflon surface coating for minimum friction.
To keep the gearbox short and compact, the two transmission shafts run above one another. The gears themselves are straight-toothed, which not only improves transmission efficiency - albeit by only a minor factor - but also helps to keep the gearbox slender and avoid extra width. Thanks to the compact structure of the transmission and the minimisation of noise from the gearbox housing, finally, running noise is reduced to a suitably low level.
Drive shaft to the rear wheel--indispensable and unique in the supersports segment.
Like all large BMW motorcycles, the K 1200 S features a drive shaft conveying power to the rear wheel. With the engine being fitted crosswise, this shaft drive configuration requires two pivot points, the pivot unit leading out of the gearbox being fitted in the gearbox cover. The loss of efficiency resulting from such double-pivotal transmission is often overestimated, since in practice it is just a few per cent. By comparison, studies confirm that as of a certain
degree of wear and contamination, chain drive suffers a significant increase in friction. This reduces the degree of efficiency accordingly, while shaft drive operates without wear and maintains a consistent standard of efficiency throughout its full service life.
The entire rear-wheel drive system is described in detail in the Suspension/Paralever Chapter.
New engine management - now featuring individual cylinder management with anti-knock control.
The K 1200 S boasts the most advanced digital engine management currently available on a motorcycle. Indeed, BMW's BMS-K (BMW Engine Management with Anti-Knock Control) Digital Motor Electronics is an in-house development specifically for motorcycles. The new generation of BMS-K technology was featured for the first time in the R 1200 GS and has now been upgraded for four-cylinder power units. Its most significant fortes are fully sequential cylinder-specific fuel injection, integrated anti-knock control, rapid processing of comprehensive sensor signals by means of the most advanced micro-electronics, compact layout, low weight, and self-diagnosis.
Introducing this sophisticated management system, BMW Motorrad is further enhancing its leadership and unique qualities already held for so many years in electronic engine management.
Momentum-based engine management with alpha-n control.
Momentum-based engine management takes a whole range of parameters into account, thus delivering torque and drive power specifically geared to running requirements and sensitively adjusting engine running conditions to all kinds of parameters. The alpha-n management principle with indirect determination of air volume intake as a function of the throttle butterfly angle and engine speed has been carried over from BMW's existing models and enhanced to an even higher standard than before. Engine speed and the throttle butterfly angle determined as before by means of a potentiometer remain the basic factors for determining the engine's operating point. Then, taking additional engine and ambient parameters (including engine temperature, air temperature, ambient air pressure) into account, the engine management system, focusing also on control maps and built-in corrective functions, tailors the volume of fuel injected and the ignition timing point to specific, individual requirements.
Fuel injection is fully sequential, meaning that fuel is injected individually into the intake ducts, precisely geared to the intake stroke in the respective cylinder.
Variable pressure control for optimum fuel supply.
Variable fuel pressure control in the supply system is another first-time achievement on a production motorcycle. The system does not have a reflow pipe or function, but rather delivers only the amount actually consumed by the engine. In practice, this highly efficient control of fuel supply allows fuel pressure to be varied almost at random for optimum fuel/air mixture formation. Fuel supply is thus tailored to actual requirements by the electrical fuel pump, applying a principle so far quite unique on a production machine and protected by patents.
The fuel/air mixture is masterminded by an oxygen sensor fitted at the junction point of the four exhaust manifolds for precise determination of exhaust emissions and their composition.
The result is even greater riding pleasure combined with even greater environmental compatibility ensured by exemplary emission management and low fuel consumption and enhanced by precise riding behaviour and a fine response at all times. BMS-K also comprises the automatic idle speed and cold start control functions. Idle speed is increased automatically whenever required in the warming-up process, controlled by an idle speed stepper (bypass ducts for additional air) integrated in the airbox and by specific adjustment of fuel injection volume.
Measuring 46 millimetres or 1.81 in diameter, the throttle butterflies come with patented operation featuring progressive adjustment. The butterfly position predetermined by the rider turning the gas handle is now controlled with even greater precision by a step motor optimising both engine response and dosage.
Incorporating various functions in one, the entire system is lighter than ever before. The three-piece injection rail made of plastic incorporates the fuel pressure sensor, while rod-shaped high-energy ignition coils housed in the cylinder head enhance the efficiency of this new engine management system.
High compression and anti-knock control for superior fuel economy.
Fuel consumption of the K 1200 S is 4.7 litres/100 km at 90 km/h and 5.5 litres/100 km at 120 km/h (60.0 and 51.4 mpg Imp, respectively). Considering the engine's power and performance, as well as its free-revving behaviour, this is truly outstanding in the supersports segment.
A major factor contributing to this superior efficiency is the very high geometric compression ratio of 13:1, a new record made possible only by anti-knock control. Following the R 1200 GS, the K 1200 S is the second BMW motorcycle to feature this protective function as standard.
Two solid body sound sensors positioned between cylinders 1/2 and 3/4 detect any knock effects in the combustion process. The engine's electronic system responds in this case by taking back (ie, retarding) the ignition angle and thus protecting the engine from possible damage. Conceived for unleaded premium plus fuel (98 RON) under normal conditions, the engine, thanks to this efficient knock control, is able to run without the slightest problem and without manual intervention on premium (95 RON) or even regular (91 RON) fue. The only difference when running on fuel of lower quality is a slight decrease in maximum output and a slight increase in fuel consumption.
Intake system - large volume for an optimum cylinder charge.
With the engine being tilted far to the front, there is space for an airbox fitting perfectly in its design and position right above the engine. The four intake manifolds lead directly into the intake system without any curvature possibly reducing their efficiency. With its volume of 10 litres, the airbox helps to boost the power of the engine and contributes to its high level of torque.
Two funnels extending straight to the front for optimum flow conditions (a configuration made possible by the location of the airbox) draw in the air required to the right and left of the headlight in the ram pressure area on the fairing. Particularly at high speeds, this ram air effect helps to further increase the cylinder charge. The intake air then passes through two separate paper filters at the end of the funnels before going into the airbox. To facilitate service and maintenance, the filters are within easy reach after removal of the fairing side sections.
Apart from the modern cyclone oil separator serving to purge air from the engine, the airbox also comprises the idle control system. And if that were not enough, it serves furthermore to hold the battery as another example of well-conceived functional integration saving both space and weight.
Exhaust system--complete with three-way catalytic converter and a sporting sound.
Four individual exhaust manifolds equal in length initially merge beneath the gearbox into two pipes and from there form one single pipe leading into the extra-large rear-end silencer (4-in-2-in-1 exhaust system).
Round in shape, the silencer has a volume of 9.5 litres and operates according to the reflection principle. The outer skin of the silencer is protected thermally by the inner absorption layer, the metal-based catalyst incorporating 200 cells/inch is fitted at the entry point of the manifolds leading into the silencer. The catalyst coating is a combination of rhodium and palladium particularly resistant to high temperatures and with a long service life.
The entire silencer system made of stainless steel weighs a mere 10.4 kilograms or 22.9 lb, making it the lightest exhaust system with a fully controlled catalytic converter in this segment of the market. Just one look at the way the manifold is fastened to the cylinder head shows the great attention to detail in the quest to save every gram.
The sporting sound of the engine, finally, ensured by careful sound engineering, maintains legal sound emission standards under all circumstances and riding conditions.
Ride Fast - Ride Safe !
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